1. Field of the Invention
This invention relates to a multiple-use thermal image transfer recording medium, capable of yielding images with high density which is maintained during the multiple use thereof.
2. Discussion of Background
Recording apparatus, such as a printer and a facsimile apparatus, using a thermal image transfer recording method, is now widely used. This is because the recording apparatus of this type is relatively small in size and can be produced inexpensively, and the maintenance is simple.
In a conventional thermal image transfer recording medium for use with the thermal image transfer recording apparatus, a single thermofusible ink layer is merely formed on a support. When such a recording medium is used for printing images, the heated portions of the ink layer are completely transferred to an image-receiving sheet at only one-time printing, so that the recording medium can be used only once, and can never be used repeatedly. The conventional recording medium is thus disadvantageous from the viewpoint of running cost.
In order to overcome the above drawback in the prior art, there have been proposed the following recording medium:
(1) A microporous ink layer containing a thermofusible ink is formed on a support so that the ink can gradually ooze out from the ink layer as disclosed in Japanese Laid-Open Patent Applications 54-68253 and 55-105579;
(2) A porous film is provided on an ink layer formed on a support so that the amount of an ink which oozes out from the ink layer can be controlled as disclosed in Japanese Laid-Open Patent Application 58-212993; and
(3) A plurality of adhesive layers and a plurality of ink layers are overlaid in turn on a support so that an ink layers can be gradually exfoliated in the form of thin layer when images are printed as disclosed in Japanese Laid-Open Patent Applications 60-127191 and 60-127192.
However, after the above-mentioned thermal image transfer recording medium (1) is repeatedly employed, the ink does not flow smoothly from the ink layer and the density of the obtained images gradually decreases.
When the diameter the pores of the porous film is increased in the case of the above-mentioned thermal image transfer recording medium (2) to increase the density of the images, the mechanical strength of the recording medium is lowered and the ink layer peels away from the support. Furthermore, in the case of the thermal image transfer recording medium (3), there is the shortcoming that the amount of the thermofusible ink contained in the ink layer which is transferred is not uniform for each printing operation.
Furthermore, most of the conventional methods have been developed in such a fashion as to be suitable for use with a serial thermal head in a recording apparatus such as a word processor. Therefore, when those methods are applied to a line thermal head for use in recording apparatus such as a facsimile apparatus and a bar code printer, problems such as the exfoliation of an ink layer, and the decrease of image density are inevitable because the time elapsed before the thermal image transfer recording medium is separated from an image-receiving sheet is relatively long after the image transfer recording medium is brought into contact with the image-receiving sheet under application of heat thereto. In the conventional thermal image transfer recording media, there has been another proposal that an intermediate adhesive layer comprising a thermofusible resin be interposed between a support and an ink layer in order to prevent the exfoliation of the ink layer from the support as disclosed in Japanese Laid-Open Patent Application 63-137891. In this case, however, heat loss during the thermal image transfer recording is increased by the provision of the intermediate adhesive layer. Accordingly, when the intermediate adhesive layer is employed for a multiple-use thermal image transfer recording medium which is thicker than the thermal image transfer recording medium which can be used only once, the thermosensitivity thereof is considerably decreased. In order to obtain satisfactory thermosensitivity, it is necessary to decrease the amount of ink coated or to increase the energy applied to the multiple-use thermal image transfer recording medium.